Twenty years ago this summer, an invasive carp was caught in the St. Croix River for the first time. In the years since, more have been shown up, raising worries the river could get infested, ruining everything from fishing to boating.

This spring, after more than three years of intensive research, a University of Minnesota fish scientist and his colleagues announced they had found a three-pronged solution to keep invasive carp from overrunning the St. Croix, as well as the upper Mississippi and Minnesota Rivers.

Now it’s up to the state legislature needs to find $8 million to make it happen. Dr. Peter Sorensen of the Minnesota Aquatic Invasive Species Center believes the projects have an “extremely high” chance of preventing the non-native fish from obliterating the ecosystem of hundreds of mile of rivers – if funded.

One of the proposal’s tactics is essentially free and ready to go with the cooperation of the U.S. Army Corps of Engineers, the operators of Lock and Dam No. 5, 10 miles upstream of Winona, MN. The scientists and the Corps are working to even out flows through the dam so a constant current is maintained that is too strong for carp to swim against.

Last line of defense

Carp can only swim so far so fast before exhausting (like people or anything else). “It’s not great, it’s very average, how far and fast carp can swim,” Sorensen says.

He saw their stamina firsthand when he traveled to Mississippi to put carp in a swim tunnel, what he described as a “huge treadmill for fish.”

The researchers took what they learned to the Army Corps of Engineers. The agency primarily manages the structure, which was built in 1935, to ensure the river stays deep enough for boat and barge traffic.

While slow water is the concern for carp fighters, areas where the current is strongest worry dam managers. Too powerful hydraulic forces could eat away at the foundation of the expensive structure.

Right now, the gates are already closed 98 percent of the time, with a small crushing flow rushing through. But in a flood, the gates are opened, which ultimately slows the water to a speed that invasive carp can swim up.

“We can see where there are weak points, where are flows the slowest, and where can the fish swim through it,” Sorensen explains. “We can see where the biggest fish can get through, and we can adjust those spots.”

Reducing peak flows and increasing low flows will not only provide a constant flow strong enough to stop carp from swimming up it, but will actually help preserve the billion-dollar structure’s footing.

“They are saying, ‘Let’s balance it out to reduce scour,’ we’re saying, ‘Let’s balance it out to keep carp from getting up there,’” Sorensen says.

Sorensen praises the Army Corps of Engineers for their cooperation in this phase. Evening out flow through the dam takes care of that route – leaving the lock.

Locking the lock

Filling and draining as needed, a lock acts like an aquatic elevator. It gives watercraft a way past the dam – and carp, too.

But based on Sorensen and his colleagues’ experiments, a combination of loud noise, lights, and bubbles should deter carp from getting anywhere near Lock No. 5’s downstream entrance.

“The lock is set to go, like it’s built for this,” Sorensen says.

Bighead and silver carp are especially susceptible to stimulation because they have a connection between their swim bladder and inner ear. It’s why silver carp leap out of the water when motorboats pass.

Sound and bubble barriers create a wall of sound, as the bubble amplifies and concentrates the sound waves. Sorensen has gotten to know invasive carp pretty well, and believes such a system would be impenetrable for the fish.

“They are very sensitive and wily,” Sorensen says. “It’s a fish that’s easy to scare but hard to catch.”

Sorensen actually thinks the wall of sound will work even better in the field than the lab. When they encounter the wall of sound, rather than swimming to other side of a tank, they’ll head downriver and probably keep going a long ways.

While native species like sturgeon and walleye don’t seem to mind the bubbles and noise, Sorensen says it would probably be best to turn the system off when carp aren’t nearby, making it less costly to operate. Underwater cameras and environmental DNA can alert managers when carp come close.

Promoting predators

The final part of the plan is using the river’s natural residents against the carp. One reason Sorensen believes carp haven’t become established in the upper Mississippi is the plethora of predators.

In fact, native fish like muskellunge and northern pike eat the eggs of invasive carp species.

Encouraging healthy populations of those predators would mean restoring habitat and possibly catch-and-release regulations. Sorensen believes it’s not just about keeping non-native fish out, but making the whole ecosystem healthier.

“I don’t want to just stop carp, I want to improve the darn river,” Sorensen says.

It would also be beneficial to bring in commercial fishermen to net invasive carp near the dam, to keep the chances low that a fish might slip through.

In the end, a healthier river has a healthier immune system. “We’ve got something to protect here, if you can protect and enhance, that’s even better,” Sorensen says.

Now or never

Even though carp have been showing up in the St. Croix River for two decades, there is still time to take action before there is a self-sustaining population. Not a single young fish has been caught in this neck of the watershed yet.

“These carp seeming like many species, they need a critical number to breed and reproduce, very social and get together in big groups,” Sorensen says. “I don’t think we have a critical number here, that’s my professional opinion.”

But we have a lot to lose if the carp take over. In areas where they are prevalent, they make up 50 percent or more of the life in the Mississippi River. Fishing isn’t much fun, and boating is dangerous.

About a decade ago, a similar strategy to the University of Minnesota’s was proposed much farther down the river. Carp were encroaching on Lock and Dam No. 19, and there was discussion of modifying the structure with bubble and sound barriers.

“It was identified in 2004 as a bottleneck for carp,” Sorensen says. “There was a huge 200 page report that suggested similar ideas, and we understand a lot more now. I’m convinced it would have worked, but no one did it, and the carp got through.”

If the same thing happens at Lock and Dam No. 5, Sorensen says there isn’t another good option for keeping them out of the St. Croix and Minnesota Rivers. Because while his proposal would use the existing lock and dam, there is no similar structure at the mouth of the St. Croix. It would be extraordinarily expensive to build something that could block the fish at Prescott.

“If my job is to stop these carp, this is what has to be done,” Sorensen says. “If there is a solution, this is it.”

Call for action

The legislature has not yet approved funding for the project this session. The state of Minnesota currently has a $900 million budget surplus! Concerned citizens can contact their local representative and senator here.